It is known to selectively convert oxygenates, including particularly methanol, to light olefins, viz. ethylene (C.sub.2.sup.=), propylene (C.sub.3.sup.=), and butylene (C.sub.4.sup.=). Ethylene and propylene are in high demand, and the need for these chemical raw materials, particularly ethylene, continues to grow. In a current process, methanol is reacted at elevated temperature over a fluidized bed of a molecular sieve catalyst, e.g., ZSM-5 zeolite or SAPO-34, to produce a reaction product from which C.sub.2 -C.sub.4 olefins are recovered.
In producing light olefins, the selectivity to ethylene can be increased to some extent by increasing reactor severity; but this has its limitations because as reactor severity is increased total olefin production is decreased. Moreover, the production of paraffins, i.e., methane, ethane, propane, etc., aromatics and other less desirable components will also increase. Attempts have also been made to modify the framework structure of the catalyst to increase light olefin production, especially the C.sub.2.sup.= /C.sub.3.sup.= ratio, while producing as little of the paraffinic and aromatic by-products as possible.
In U.S. Pat. No. 3,911,041, there is disclosed a process for converting methanol to a reaction product containing light olefins by contact of the methanol with a phosphorus modified zeolite. A zeolite of intermediate pore size, such as ZSM-5, is modified by incorporating from about 0.78 wt. % to 4.5 wt. % phosphorus bonded to its structural framework. Typically the dry ZSM-5 zeolite is contacted with a solution of a phosphorus-containing compound, e.g., PCl.sub.3, and heated at elevated temperature for a time sufficient to incorporate the phosphorus within the crystalline framework of the zeolite. In conducting a methanol to olefins reaction with a phosphorus modified ZSM-5 catalyst, as demonstrated by the Patentee, selectivities at 325.degree. C. to the C.sub.2 -C.sub.3 olefins are high compared to the selectivities to the C.sub.2 -C.sub.3 paraffins, but butane production was significant. At temperatures above 325.degree. C. selectivities to the C.sub.2 -C.sub.4 olefins were high compared to the selectivities to the C.sub.2 -C.sub.4 paraffins. At temperatures above 500.degree. C. the concentration of ethylene in the product increased, but propylene remained the major component relative to ethylene. At temperatures below 500.degree. C., at conditions which increased the ratio of ethylene:propylene, the production of butenes, butanes, or both butenes and butanes was increased significantly; sufficiently so that the molar ratio of C.sub.2.sup.= /C.sub.4 &lt;1. Changing the severity of a reaction to change the distribution of olefins and by-products thus has its limitations.
Other modifications of zeolite catalysts have been proposed, or made, but there remains a pressing need for improving the performance of this type of catalyst for selectively producing light olefins, particularly ethylene, from oxygenates, notably alcohols, with low by-products formation.